The processing of waste including municipal waste, medical waste, toxic and radioactive waste by means of plasma torch based waste processing plants is well known.
Due to the high temperatures generated in processing plants by plasma torches, various cooling means are necessary to prevent localized overheating which can have detrimental effects on the components of the plant. One area that requires cooling is the opening in the plant chamber wall that is located typically at the lower part thereof to facilitate installation and removal of the plasma torch. A gap separates the outer surface of the plasma torch that is inserted through the opening, from the surrounding chamber wall. In order to prevent heat damage to the outside metal shell of the chamber wall, caused by heat radiating through the gap from inside the chamber, a water cooled shield is typically provided on the outside surface of the chamber in proximity to the plasma torch that is installed in the opening.
After several hours of running time of the processing plant, the inner surface of the lower part of the chamber may reach temperatures of up to 1800-2100 K. Despite the gap that separates the plasma torch from the surrounding chamber wall, the body of the plasma torch absorbs the heat that radiates from the chamber wall. This causes the temperature of the outer wall of the plasma torch to rise, which decreases the efficiency of the process. Additionally, this can result in shortening the life of the plasma torch. The plasma torch is usually cooled by a suitable liquid coolant such as water in order to prevent damage to the plasma torch. This coolant must be capable of removing heat build-up not only as a result of normal operation of the torch, but as a result of radiation from the surrounding chamber wall as well.
The size of the gap is one of the factors in determining the amount of heat losses from the processing chamber. Reducing the gap allows less heat to radiate out of the chamber, thereby reducing the heat losses from the chamber, as well as potential damage to the outside of the chamber. In addition to the width of the gap, heat losses are further dependent on the temperature difference between the inside of the chamber and the cooled outside of the chamber wall and the outer surface of the plasma torch.
Another problem related to the operation of a plasma torch is caused when the plasma forming gas that is used is air. Although air is the least expensive gas that may be used to produce the high temperature plasma jet, the use of air leads to a relatively short life for the plasma torch due to high temperature oxidation of the metal components of the torch.
When air is utilized as the plasma forming gas of the plasma torch, large quantities of hot oxidizing gas enter the chamber. However, air is composed of mostly nitrogen, which dilutes the product gasses and decreases its ability to yield a high calorific value. Therefore, steam is often used as an additional oxidizing gas. However, since it is problematic to use steam as the plasma forming gas in the plasma torch, the steam is generally fed at low temperatures to the chamber.
If the temperature of the oxidizing agent that is provided to assist in oxidizing organic material of the treated waste is low, it may lead to cooling the location near the inlet of the oxidizing agent, and to the appearance of abnormalities in the movement of the waste through the chamber. These abnormalities may further lead to larger problems in the lower part of the chamber such as congestion of the apparatus and increasing the viscosity of the molten material, as well as problems in the shaft, such as bridging, i.e. a blockage in the form of a bridge that occurs as a result of the creation of solid material in the chamber.
U.S. Pat. No. 5,695,662 discloses a plasma arc torch that is utilized to cut sheet metal such as thick plates of steel, thin plates of galvanized metal, etc. When the piercing begins, prior to the metal being cut through, the molten metal is splashed upward onto the torch. This is undesirable because it can destabilize the arc, causing it to gouge the nozzle, which can reduce the life of the nozzle, or even destroy it. Therefore, U.S. Pat. No. 5,695,662 provides a high velocity flow of an oxygen rich secondary gas mixture around the nozzle to form a cold layer of gas that is used as a shield to protect the nozzle and other torch components adjacent to the workpiece from splattered molten metal. Additionally, using an oxygen rich secondary gas mixture improves the piecing capabilities of the torch by allowing a cleaner and deeper penetration into the metal than torches utilizing other gas mixtures. The secondary gas is introduced at the upper end of the torch, travels through the torch body toward the nozzle, passes through a ring having an array of off-center slits, thereby introducing a swirling movement to the flow, and exits the torch in a swirling flow immediately adjacent to the plasma arc. However, since the plasma cutter is generally not situated in an enclosed, heat radiating environment, the detrimental effect caused by external heat radiating on the outer surface of the plasma torch is not present. Therefore, U.S. Pat. No. 5,695,662 does not relate to providing means for cooling the longitudinal outer surface of a plasma torch.
U.S. Pat. No. 3,949,188 discloses an arc transfer torch having a cathode rod and two coaxial annular bodies. An inactive gas is supplied in the annular space between the cathode rod and the first annular body, establishing an arc between the cathode rod and a piece of metal to be cut. An active gas is supplied in the annular space between the first and second annular bodies, establishing plasma composed of the active gas that is heated at a high temperature. According to U.S. Pat. No. 3,949,188, heat losses at the nozzle aperture of the second annular body decrease if the flow rate of the inactive gas is decreased below a certain critical value. U.S. Pat. No. 3,949,188 does not relate to any method of cooling the longitudinal outer surface of the plasma torch at all, and only relates to cooling the nozzle of the second annular body by cooling water that is supplied thereto.
U.S. Pat. No. 5,514,848 discloses a plasma torch having cylindrical symmetry. The internal passage between the cathode and anode is shaped to include a restriction that accelerates the follow of the plasma gas introduced at the cathode end. According to the inventors the result of the restriction is to increase the arc length, while allowing a lower amperage to voltage ratio for a given power input. The part of the torch between the cathode assembly and anode is surrounded by a coaxial cylinder that forms a cooling chamber through which a cooling fluid, which enters the chamber via an inlet at the bottom (anode) end of the torch and exits through an outlet at the top (cathode), is circulated.
It is therefore an aim of the present invention to provide a plasma torch arrangement that overcomes the limitations of prior art arrangements.
It is another aim of the present invention to provide such an arrangement that introduces a preheated oxidizing medium to the processing chamber of a plasma waste processing plant
It is another aim of the present invention to provide such an arrangement that minimizes heat losses in a plasma waste processing plant.
Other purposes and advantages of the present invention will appear as the description proceeds.